Pioneer neural network research was conducted at the former Atmospheric Sciences Laboratory in the early 1990's (Measure & Yee, 1992). The research involved experimentation with neural network methods to retrieve temperature profiles from ground based microwave radiometers (Yee & Measure, 1992) as well as from satellite radiance measurements (Bustamante, et al, 1994). Neural networks were trained using simulated microwave radiometric measurements and archived radiosonde measurements to produce vertical profiles of temperature from the surface to approximately 10 kilometers.
The success of these earlier studies prompted wind vector retrievals using satellite radiances (Cogan, et al, 1997). Those experiments have yielded errors comparable to those achieved by other sounder based methods. Current studies involve the fusion of varied measurement sources to improve the upper level wind retrievals using neural network techniques. Neural networks are ideally suited for processing diverse data measurements and analyzing large data sets.
One of the salient features of wind profilers, in general, is that they can provide continuous time measurements without the extra expenditure of resources that a radiosonde would require. The disadvantages or limitations of wind profilers include limited range during adverse weather conditions, interference from insects and birds, false signals from other sources, and incomplete or missing data coverage.
Wind Profiler Systems
There are several wind profiler systems that are available in the commercial market. The first, is the Sodar (sonic detection and ranging) system. Sodar systems are used to remotely measure the vertical turbulence structure and the wind profile of the lower layer of the atmosphere. Sodar systems are like radar (radio detection and ranging) systems except that sound waves rather than radio waves are used for detection. Other names used for sodar systems include sounder, echo sounder and acoustic radar. A more familiar related term may be sonar, which stands for sound navigation ranging. Sonar systems detect the presence and location of objects submerged in water (e.g., submarines) by means of sonic waves reflected back to the source. Sodar systems are similar except the medium is air instead of water and reflection is due to the scattering of sound by atmospheric turbulence.
Most sodar systems operate by issuing an acoustic pulse and then listening for the return signal for a short period of time. Generally, both the intensity and the Doppler (frequency) shift of the return signal are analyzed to determine the wind speed, wind direction and turbulent character of the atmosphere. A profile of the atmosphere as a function of height can be obtained by analyzing the return signal at a series of times following the transmission of each pulse. The return signal recorded at any particular delay time provides atmospheric data for a height that can be calculated based on the speed of sound. Sodar systems typically have maximum ranges varying from a few hundred meters up to several hundred meters or higher. Maximum range is typically achieved at locations that have low ambient noise and moderate to high relative humidity. At desert locations, sodar systems tend to have reduced altitude performance because sound attenuates more rapidly in dry air.
Another type of wind profiler system is the Radar (radio detection and ranging) system. The Radar systems are similar in principle to sodars except that radio frequencies are transmitted instead of sound waves. These systems tend to have longer ranges than the sodars but can be very large physically and not as mobile.
Yet another type of wind profiler system is the Lidar (light detection and ranging) system. The Lidar systems use Doppler frequency shifts in the light region of the electromagnetic spectrum. Typically, visible and infrared wavelengths of light are employed in these systems. One disadvantage in these systems is that infrared radiation can be absorbed under wet atmospheric conditions.
Some of the advantages of wind profiler systems are obvious compared to erecting tall towers with in-situ wind and temperature sensors. First, a wind profiler system can generally be installed in a small fraction of the time it takes to erect a tall tower except for large radar antenna systems. Sodar systems do have some drawbacks compared to tall towers fitted with in-situ wind sensors. One of the most significant is the fact that sodar systems generally do not report valid data during periods of heavy precipitation.
All the wind profilers have certain limitations and atmospheric conditions play a very important role in the retrieval of reliable wind vectors. In many cases, it is difficult to obtain consistent winds at the maximum detectable heights of these remote sensors. A neural network has been developed to estimate upper level winds from these ground based wind profilers to extend their capabilities at a particular locale.
Radiosondes
While various efforts were attempted at remotely sensing the atmosphere with instruments onboard unmanned free balloons, the current type of radiosonde dates back to January 1930, when Pavel A. Molchanov, a Russian meteorologist, made a successful radio sounding into the stratosphere. He launched his radiosonde at Pavlovsk. His goal was a cheap, and expendable means of sounding the atmosphere for temperature, moisture and wind data.
Radiosondes were first used by the U.S. Weather Bureau in 1936. During that year a radiosonde network of several stations was inaugurated to obtain upper air soundings on a routine basis. This network replaced the kite and aircraft sounding programs. Currently, 70 radiosonde stations are distributed across the continental United States. Radiosondes are launched from these stations twice daily, just prior to 0000 and 1200 universal time. Radiosondes can be launched in almost any type of weather. While the radiosonde is reasonably durable, severe thunderstorms and heavy precipitation may cause instrument failure or radio interference.
Weather balloons measure the upper air at heights from near the ground up to 30 km. Weather balloons carry instrument packages called radiosondes high into the atmosphere that gather essential upper-air data needed to forecast the weather. These instruments are launched twice a day at 1,100 sites around the world. Temperature, humidity and air pressure are measured at various altitudes and transmitted via radio waves to a receiving station. Radio navigation supplies wind speed and direction at each altitude.
The biggest disadvantages of radiosondes are expense and timeliness, the lack of continuous measurements. Typically, radiosondes are launched twice a day: one in the morning and one in the evening. This does not cover the whole day in which significant weather events can occur. In terms of expense, once a weather balloon is launched, the balloon, the meteorological sensors, and the helium used to inflate the balloon are lost.
What are needed are methods and systems to obtain near continuous, real-time wind measurement data.